In a gas turbine and steam turbine installation or system, the heat contained in the expanded working fluid or exhaust gas from the gas turbine is used for generating steam for the steam turbine connected into the water/steam cycle. In this arrangement, the heat transmission takes place in a waste-heat steam generator or boiler which is connected downstream of the gas turbine and in which heating surfaces are arranged in the form of tubes or tube bundles. The latter are in turn connected into the water/steam cycle of the steam turbine. In this arrangement, the water/steam cycle usually includes a plurality of pressure stages, for example two or three pressure stages, with a preheater, an evaporator and a superheater being provided as heating surfaces in each pressure stage. Such a gas turbine and steam turbine installation is, for example, known from EP 0 523 467 B1.
In this arrangement, the total water quantity routed within the water/steam cycle is dimensioned in such a way that, because of the heat transmission, the exhaust gas leaving the waste-heat steam generator is cooled to a temperature of approximately 70° C. to 100° C. In particular, this indicates that the heating surfaces exposed to the hot exhaust gas and pressure drums provided for water/steam separation are designed for full-load operation or rated operation, at which an installation efficiency of approximately 55% to 60% is currently achieved. For thermodynamic reasons, an attempt is then also made to ensure that the temperatures of the feed water routed within the individual heating surfaces and at different pressures are as close as possible to the temperature variation of the exhaust gas, which is being cooled along the waste-heat steam generator as a consequence of the heat exchange. The objective is then to keep the temperature difference between the feed water routed over the individual heating surfaces and the exhaust gas as small as possible in each region of the waste-heat steam generator. In order to transfer, in this process, the highest possible proportion of the heat quantity contained in the exhaust gas, a condensate preheater is additionally provided in the waste-heat steam generator in order to heat condensed water from the steam turbine.
The gas turbine of such a gas turbine and steam turbine installation can be designed for operation with different fuels. If the gas turbine is designed for heating oil and natural gas, the heating oil is only provided as the gas turbine fuel for a short operation duration, for example for between 100 and 500 hours per annum, as so-called back-up for the natural gas. In this arrangement, the gas turbine and steam turbine installation is usually designed and optimized, as a priority, for natural gas operation of the gas turbine. The necessary heat can be extracted in various ways from the waste-heat steam generator itself in order then to ensure a sufficiently high inlet temperature of the condensate flowing into the waste-heat steam generator in the case of heating oil operation, particularly in the case of a change from gas operation to oil operation.
One possibility resides in completely or partially bypassing the condensate preheater and heating the condensate, in a feed-water tank connected into the water/steam cycle, by supplying low-pressure steam. At low steam pressures, however, such a method requires a large-volume, and under certain circumstances multistage, heating-steam system in the feed-water tank which, in the case of large heating ranges, can endanger a degassing system which is usually located in the feed-water tank.
In order, in particular, to ensure effective degassing of the condensate, the condensate temperature in the feed-water tank is usually kept within a temperature range between 130° C. and 160° C. For this purpose, the condensate is usually preheated by a preheater fed with low-pressure steam or hot water from an economizer, so that the heating range of the condensate in the feed-water tank is kept as small as possible. Particularly in the case of two-pressure or three-pressure installations, extraction of hot water from the high-pressure economizer is then necessary in order to make sufficient heat available. Particularly in the case of three-pressure installations or circuits, however, this has the substantial disadvantage that an additional, external condensate preheater is required which must be designed for the high pressures and high temperatures and/or high temperature differences. This method is therefore extremely undesirable simply because of the substantial costs and the additional space requirements for the condensate preheater.
In the case of oil operation of the gas turbine, it is also possible to undertake or support the condensate heating in the feed-water tank or in the degasser with a partial flow of steam supplied from a reheater. This method, however, is not applicable either, particularly in the case of modern installation circuits without feed-water tank or without degasser, particularly since corresponding appliances or apparatus for mixed preheating are lacking.
A method which can be carried out with less equipment and operational complexity, as compared with the methods described, is known from DE 187 36 889 C1. This method is based on a displacement of exhaust gas heat in the direction of condensate preheating as a consequence of a reduction in the low-pressure region and of an installation of bypasses on the water side of the economizer. This method, however, also meets realization limits in the case of certain requirements.